README: add note about Dockerfile

README.rst

@@ -501,13 +501,14 @@ Running in Docker
 =================
 
 Also you are able to run and use Salt inside Docker_ container on Linux machine.
-Let's prepare Docker image with both Salt Master and Minion installed by the bootstrap script:
+Let's prepare the Docker image using provided ``Dockerfile`` to install both Salt Master and Minion
+with the bootstrap script:
 
 .. code:: console
 
   docker built -t local/salt-bootstrap .
 
-Start your new container with Salt services:
+Start your new container with Salt services up and running:
 
 .. code:: console
 
@@ -522,8 +523,15 @@ And finally "enter" the running container and make Salt fully operational:
 
 Salt is ready and working in the Docker container with Minion authenticated on Master.
 
+.. note::
+
+  The ``Dockerfile`` here inherits Ubuntu 14.04 public image with Upstart configured as init system.
+  Consider it as an example or starting point of how to make your own Docker images with suitable
+  Salt components, custom configurations and even `pre-accepted Minion key`_ already installed.
+
 
 .. _Docker: https://www.docker.com/
+.. _`pre-accepted Minion key`: https://docs.saltstack.com/en/latest/topics/tutorials/preseed_key.html
 
 
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